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5 5 Background The sensing area of each nodes is represented by a circle with the same radius. All sensor nodes know their locations. Sensors can plan paths from their current position to a desired destination. How can we place a certain number of additional circles (the sensing circle of the mobile sensors) to maximize the overall coverage?

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7 7 Background Voronoi Cell (cont.)  If sensor cannot detect the expected phenomenon in its Voronoi polygon, no other sensor can detect it.  Each sensor only needs to check its own Voronoi polygon. If its sensing area cannot cover the polygon, there are some coverage holes.

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10 Bidding Protocols Overview (cont.)  The advertisement phase ： Each mobile sensor has a base price, which is related to the size of any new hole generated by its movement. Mobile sensors broadcast their base prices and locations in a local area.  The bidding phase ： Examining the service list, the static sensor chooses a mobile sensor whose base price is lower than its bid and sends a bidding message to this mobile sensor.

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11 Bidding Protocols Overview (cont.)  The serving phase ： The mobile sensor chooses the highest bid and moves to heal that coverage hole. After the serving phase, the mobile sensors broadcast their new locations and new base prices and a new round begins.  Termination When no static sensors can give out a bid higher than the base price of the mobile sensors.

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12 Bidding Protocols Distributed Calculation of the Voronoi Cell  Static sensors construct Voronoi cells considering only static neighbors and mobile neighbors which are not likely to move.  If the base price of a mobile sensor is zero, this mobile sensor has not moved yet and most likely it will move to heal some coverage hole.

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13  Static sensors calculate the bid as Where d is the distance between the bidder and the target location. Bid Estimation  Inside one coverage hole, there are many positions at which a mobile sensor can be located.  The static sensor chooses the farthest Voronoi vertex as the target location of the coming mobile sensor. Bidding Protocols Bid estimation

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14 Bid Estimation (cont.)  Using the greedy heuristic to choose the largest coverage hole may not be optimal in some cases.  We propose an optimization which puts a limit on the maximum distance. This maximum distance, denoted by. We choose to be  Note that the maximum base price is Bidding Protocols Bid estimationCircle Covering

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17 Bidding Protocols Multiple Healing Detection  Due to the limited communication range, static sensors may have independently bid different mobile sensors for the same coverage hole. If it finds out that some other mobile sensors have a higher base price than its own, it will run the detection algorithm to check whether a multiple healing has occurred. The duplicate healing problem.

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20 This protocol improves the performance of the basic bidding protocol in terms of energy efficiency and load balance. Proxy-Based Bidding Protocol

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21 In this protocol, sensors only move after their final location is determined. The key trade-off is the increased number of messages versus the decreases in required movement. Proxy-Based Bidding Protocol

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23 Proxy-Based Bidding Protocol Proxy Sensor  The first problem to be addressed is how to advertise services to the neighborhood of those virtual positions when mobile sensors do not move. 23

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24 Proxy Sensor  The first problem to be addressed is how to advertise services to the neighborhood of those virtual positions when mobile sensors do not move. Proxy-Based Bidding Protocol 24

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25 Coverage Hole Exchange  Coverage hole exchange is proposed to reduce the overall moving distance and to reduce the chance that an individual sensor is penalized by moving a long distance. Proxy-Based Bidding Protocol

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26 Coverage Hole Exchange (Cont.)  All exchanges between si and sj must satisfy the following prerequisites; otherwise, the exchange will not be performed ： Proxy-Based Bidding Protocol  There is an exception to this general principle. For very large holes, i.e., holes bigger than the sensing range of a single sensor. 26

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27 Multiple Healing Detection  A proxy sensor calculates the Voronoi cell without considering its mobile sensor, as if its bid in the previous round had failed. Proxy-Based Bidding Protocol Then, it checks whether the original coverage hole remains; if the same hole exists, no multiple healing has occurred.

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28 Protocol Specification  Advertisement phase ： Proxy sensors advertise the virtual locations, physical locations, and base prices for their delegated mobile sensors. In the first round, a mobile sensor does not have a proxy and advertises its physical location and base price by itself. Proxy-Based Bidding Protocol

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29 Protocol Specification (Cont.)  Bidding phase ： Static sensors calculate their Voronoi cells based on the virtual positions of mobile sensors. Estimate the hole size, choose the closest or cheapest mobile sensor, and send bidding messages to its proxy or the mobile sensor itself if the mobile sensor has no proxy. Proxy-Based Bidding Protocol

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31 Protocol Specification (Cont.)  Hole-exchange phase ： Proxy sensors check the received requests, choose one with the highest priority, and return the confirm message to the requester.  Termination When all the largest holes are healed and no more hole exchanges are necessary. Proxy-Based Bidding Protocol

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33 Metrics  In a 60 m * 60 m flat field.  The transmission range is set to be 20 meters.  The sensing range is set to be 6 meters.  We randomly distribute 60 sensors.  We assign a percentage of sensors to be mobile.  We run 50 experiments based on different initial distributions and calculate the average results. Performance Evaluations

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34 Performance Evaluations The number of sensors needed to reach certain coverage under different mobile percentage.

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39 Conclusion We identified the problem of deploying mobile sensors in a mixed sensor network as an NP-complete problem and designed bidding protocols to tackle this problem in a distributed fashion. The proxy-based protocol improves the performance of the basic bidding protocol in terms of energy efficiency and load balance.